A silicon nitride sintered body obtained by molding and heating/sintering a silicon nitride powder is excellent in high strength, corrosion resistance, thermal impact resistance, thermal conductivity, electrical insulation and the like, and therefore being used, for example, as a wear-resistant member such as cutting tip and ball bearing, a high-temperature structural member such as automotive engine component, and a circuit substrate. The silicon nitride sintered body is usually produced by mixing a sintering aid with a silicon nitride powder, subjecting the mixture to press molding, injection molding, extrusion molding or the like to form a compact, and sintering the compact.
The method for obtaining a silicon nitride sintered body having a high mechanical strength includes, for example, the method of Patent Document 1. Patent Document 1 discloses a production method including pyrolyzing an amorphous silicon nitride powder and/or a nitrogen-containing silane compound, wherein a silicon nitride powder with the internal oxygen amount and surface oxygen amount being adjusted to specific ranges is obtained by controlling the oxygen amount in the amorphous silicon nitride powder and/or nitrogen-containing silane compound and the oxygen partial pressure in the firing (pyrolysis) atmosphere. It is stated that the bending strength of a silicon nitride sintered body produced using the silicon nitride powder above shows a high value both at room temperature and at 1,200° C. In the production method of Patent Document 1, the surface oxygen amount of the silicon nitride powder can be adjusted to a range suitable for sintering, but the method does not achieve success in adjusting the surface oxygen amount to a range suitable for sintering and at the same time, reducing the internal oxygen amount.
On the other hand, Patent Document 2 discloses a direct nitridation method including heating a metallic silicon powder in a nitrogen gas atmosphere or a nitrogen-containing non-oxidizing gas atmosphere, wherein a silicon nitride powder more reduced in the internal oxygen amount than in the silicon nitride powder of Patent Document 1 is obtained by controlling the oxygen content of the raw material metallic silicon powder and the amount of water in the atmosphere above. However, this silicon nitride powder is produced by a direct nitridation method, and therefore not only a pulverization step is required but also a silicon nitride sintered body obtained by sintering the silicon nitride powder above fails to have a high mechanical strength. In using a silicon nitride powder produced by a direct nitridation method for the raw material of a sintered body, the powder must be pulverized as described above, which makes it difficult to obtain a powder having both appropriate particle size distribution and specific surface area to enable increasing the sintering density, and moreover, a part of an acid used for removing an impurity mixed during pulverization unavoidably remains in the silicon nitride powder. Furthermore, in a direct nitridation method, the raw material metallic silicon (metal silicon) is likely to remain inside a silicon nitride particle constituting the silicon nitride powder and often gives rise to production of a pore or a coarse particle in the inside of a silicon nitride sintered body, and this is also the reason.
The silicon nitride sintered body is used not only as a structural member but also as a circuit substrate, and a silicon nitride sintered body having, among others, a high coefficient of thermal conductivity, in addition to high mechanical strength, is required. Patent Documents 1 and 2 are silent on the coefficient of thermal conductivity of the silicon nitride sintered body or use of the silicon nitride sintered body for a circuit substrate, but a silicon nitride powder suitable for the production of a silicon nitride sintered body having in particular a high coefficient of thermal conductivity as well as high mechanical strength is demanded.